1. Field of the Invention
The present invention relates to the calibration of nuclear density gauges. More particularly, it relates to the recalibration of such gauges after a period of use.
2. Description of the Prior Art
Nuclear radiation gauges for determining density characteristics of soil and asphaltic materials are well known. One example of such a gauge is described in U.S. Pat. No. 2,781,453. Such gauges employ the phenomenon of Compton scattering of gamma rays and are well known to those skilled in the art as "scatter" gauges.
Nuclear density gauges currently in use, for example Troxler Model No. 3411-B manufactured by the assignee of the instant invention, employ a source that emits gamma radiation into the test specimen and a detector for accumulating counts of scattered radiation. The gauge typically rests on the surface of the test specimen. The source is vertically movable from a "backscatter" position where it resides within the gauge housing to a plurality of "direct transmission" positions where it is inserted at selected depths into bores in the test specimen .
As an industry standard, the counts received by the detector are related to the density of the test specimen by a working exponential equation containing three constants A, B, C. The equation may take the form EQU CR=A exp(-BD)-C,
where:
CR=count ratio (the accumulated count normalized to a reference standard count for purposes of eliminating long term effects of source decay and electronic drift) and PA1 D=density calculated by the gauge. PA1 CR.sub.1 '=the count ratio from step (a), and PA1 D.sub.1 '=the known density of the first calibration block. PA1 CR.sub.2 '=the count ratio from step (b), and PA1 D.sub.2 '=the known density of the second calibration block. With the assumption that constant B does not change in any statistically meaningful way throughout the useful life of the gauge, the equations (i) and (ii) may be solved to give values for the new calibration constants A' and C' for the first source depth position. The gauge may then be repositioned with the source at the remaining source depth positions of the gauge and steps A, B and C reexecuted for each remaining position. According to this method the gauge is accurately recalibrated for each source depth position with the use of only two density calibration blocks and a computing device capable of exponential functions.
The gauges are factory calibrated to arrive at values for constants A, B, C for each gauge.
The factory calibration is achieved by the accumulation of count data on at least three standard density calibration blocks. Accumulation of count rate data on other calibration blocks may be taken for the purpose of taking into account the composition of the soil and ashpaltic test specimens; however, in any event at least three blocks must be used to derive the three constants.
In normal use nuclear density gauges undergo stress that can change the geometry of the gauge. Changes in geometry, as well as other factors, affect the gauge calibration accuracy such that after a period of time there is a need for recalibration of the gauge to arrive at new values for the constants A, B, C. The standard practice in the industry has been for the gauge user to return the gauge to the factory, or to a regional recalibration center, where the factory calibration process is repeated.
The mentioned factory calibration process involves a rigorous iterative calculation process that, for all practical purposes, must be performed on a computer. Thus, there is a pressing need for a simplified recalibration technique, preferably one that can be performed in the field with a miniumum of equipment.